Reaction Injection Molding (RIM) is a process involving filling a closed mold with highly reactive liquid starting components within a very short time to produce a rigid microcellular product having a continuous outer skin. The RIM process is important in the production of external automotive body components.
The RIM process involves the mixing of a polyisocyanate component with a isocyanate-reactive components and simultaneous injection of this mixture into a mold for subsequent rapid curing. The polyisocyanate component is typically based on a liquid polyisocyanate. The isocyanate-reactive component contains a high molecular weight isocyanate-reactive component, typically a polyol and/or an amine polyether and usually contains a chain extender containing amino and/or hydroxyl groups.
The properties of the resulting product are dependent on a large number of variables such as the nature of the starting liquid components, the amount and quality of nitrogen dissolved in the isocyanate-reactive component (nucleation) and the amount and character of other additives which may include a variety of fillers. Such fillers may include materials such as fibreglass, mineral fillers or solid and/or hollow microspheres of a glass or ceramic material.
The RIM process is a complex process. It is sensitive to reaction conditions such as the presence and type of nucleating agents, the liquid reactants etc. Moreover, the product produced, especially for exterior automotive applications, have stringent requirements. Automotive fascias must have mechanical properties in order to pass the required strength, thermal, impact, durability tests etc. Exterior automotive components must also have a highly decorative finish provided by high gloss painting. For example, a fascia for an automobile must have a continuous high gloss outer skin, be lightweight and pass impact tests as required in various jurisdictions.
Fascias and other automotive products can be provided by other less expensive molding processes such as from thermoplastic olefin materials. Generally the advantages of such materials is in price and density reduction rather than quality. An advantage of RRIM/RIM moldings have over thermoplastic materials is the ability to sand the surface of the molded product to remove all visible parting lines without producing surface defects. The problems encountered with other low density fillers producing low density RRIM/RIM products for automotive exteriors was the presence of pitting on the surface of the painted part at the sanded areas. This pitting is due to the breakage of the low density fillers. Therefore, there is a considerable advantage to produce a RRIM/RIM automotive fascia which maintains the surface and physical qualities of RIM products but which has a density and price comparable with thermoplastic olefin materials.
Due to the weight and cost advantages of thermoplastic olefin materials for fascia products, there has been considerable previous work done in the efforts of decreasing both cost and density of polyurethane products.
For example U.S. Pat. No. 5,244,613 issued Sep. 14, 1993 to Hurley et al. discloses the use of expanded organic microspheres encapsulating a blowing agent as means of reducing the density of a rigid RRIM molding. U.S. Pat. No. 3,866,653 issued 1975 to Ahmad discloses the use of hollow glass or ceramic microspheres in an elastomeric polyurethane to be used in the cavity of a pneumatic tire. U.S. Pat. No. 4,839,393 issued Jun. 13, 1989 to Buchanan et al. This patent discloses the use of glass beads and/or bubbles to be mixed with polyurethane foam chips as a filler to increase the volumes of foams while controlling the lightness in weight. These polyurethane foams are not of the rigid type as utilized for automotive exterior components. U.S. Pat. No. 4,539,345 issued Sep. 3, 1985 to Hansen discloses the use of glass bubbles as a filler for moisture curable polyurethane compositions to be used as adhesives, coatings, sealants or casting resins.
More recent research into the production of a rigid RIM product has resulted in a modified chemical system to produce polymers of greater strength and incorporating, as dual fillers, hollow ceramic microspheres known to cause pitting and wollastokup, a reinforcing filler with poor impact properties.